1. Field of the Invention
The present invention relates to a data transmission method in an optical star network used in, for example, an optical LAN (Local Area Network), and an optical star network system that realizes the method.
2. Description of the Related Art
Recently, optical LANs are being put to practical use in different fields, owing to the broad band characteristic and electromagnetic interference proof characteristic of optical fibers which serve as optical paths. Of these optical LANs, there is one which has been receiving attention due to high reliability of its components and its capability to provide a complex system by means of wave multiplex. This type utilizes a passive optical star coupler, serving as a center node, whose optical path can be constituted entirely by passive parts, and a CSMA/CD (Carrier Sense Multiple Access/Collision Detection) as an access system.
FIG. 1 exemplifies a prior art system of this type, which can perform data transmission and video signal transmission. In this system, optical fiber transmission paths 4 radially extend from optical star coupler (SC) 2 serving as a center node. A plurality of data terminals (DTE) 14 and 20 are coupled through respective optical transceivers (OTR) 16 and 18 to these optical transmission paths 4. Camera 6 and monitor device 12 are coupled to optical star coupler 2 respectively through optical transmitter (OT) 8 and optical receiver (OR) 10. Further, controller (CNT-R) 310 for camera 6 and another controller (CNT-T) 316 for giving a control command to controller 310 are coupled to optical star coupler 2 through optical transceiver (OTR) 306 and adapter (ADP) 308, and OTR 320 and ADP 318, respectively.
In thus constructed optical LAN, data transmission between DTEs 14 and 20 is performed according to the CSMA/CD system. When it is confirmed that no data transmission from other data terminals, for example, DTE 14 generates a data packet (main data packet) and performs Manchester conversion of the data packet. The converted packet is sent to OTR 16 for another Manchester conversion (double Manchester conversion). Then, it is modulated to be an optical signal and is sent to optical star coupler 2. In contrast, OTR 18 subjects the data packet optical signal coming through optical star coupler 2 first to partial response conversion and then reproduces a Manchester code. This reproduced Manchester code is sent to DTE 20 where the data packet is reproduced. If data packets are simultaneously transmitted from two or more DTEs, there would be a collision of optical signals corresponding to the data packets in optical star coupler 2 or on optical paths 4. This collision is detected as failure in coding rule of Manchester conversion by the individual OTRs. When collision of optical signals is detected, each OTR temporarily stops transmitting the optical signal at that moment and starts transmitting the optical signal again after a given time In this case, when another optical signal has already been transmitted, each OTR starts transmitting the optical signal upon completion of transmission of another optical signal.
In transmitting a video signal from camera 6 to monitor device 12, the video signal attained by camera 6 is modulated by OT 8 to be an optical signal of a waveband different from that of the aforementioned main data packet and is sent onto optical transmission paths 4. OR 10 on the side of monitor device 12 photoelectrically converts the optical video signal coming through optical star coupler 2 into a video signal. This reproduced video signal is supplied to, and displayed on, monitor device 12. At this time, although OR 10 receives a main data packet optical signal, etc. besides the optical video signal, such an optical signal as the main data packet is eliminated by a filter Therefore, monitor device 12 displays only the video signal from camera 6, which ensures a local area monitoring, using an ITV camera, for example.
In effecting a local area monitoring using the above optical LAN system, it is necessary to remotely control the visual field or the like for image pickup of a camera by sending control data to the camera side from the monitoring device side. This control data is conventionally sent to controller 310 from controller 316 as follows. As well as data transmission between DTEs 14 and 20, first, OTR 320 checks whether or not another OTR or OT in the system is transmitting an optical signal in accordance with the CSMA/CD system. When it is determined that no optical signal is being transmitted, the control data generated from CNT-T 316 is into a packet in ADP 318 and is converted, by OTR 320, into an optical signal which is then sent on optical transmission paths 4. On the side of camera 6, by way of contrast, the optical signal coming via optical transmission paths 4 is subjected to photoelectric conversion in OTR 306 and the control data is then reproduced from the optical signal by ADP 308. CNT-R 310 controls the visual field or the like of camera 6 in accordance with this control data.
With such a prior art system, however, since the control data is transmitted according to the CSMA/CD system as well as data transmitted between DTEs 14 and 20, various devices used for control data transmission become complicated and enlarged. This accordingly enlarges, and increases the cost of, the network system.